Metals, especially different steels and their alloys, and more specifically high speed steels (HSS) are commercially used in cutting tool industries as structural members or tools as also in engineering industries in applications widely ranging from kitchen knives to turbine components. Use of these materials is dictated by the cost and strength requirement. Though available in abundance at affordable prices, these materials have certain limitations such as less wear resistance, its degree depending on the percentage of carbon and other alloying elements.
Heat treatment of steels is a well-known process of hardening, which is in use for the last 100 years or more. The hardened layer provides high-level protection against wear, tear and corrosion. Various hard facing techniques such as welding, laser hardening, plasma spraying, high velocity oxy fuel spraying, detonation spraying have been well developed and widely used by engineering industries to produce hard wear resistant coatings. Thin coating techniques such as physical vapor deposition (PVD) and chemical vapor deposition (CVD) are also developed for improving service life of cutting tools and other engineering components.
All these coatings are essentially employed on components, made of different metals and alloys, to combat various forms of wear, tear and corrosion to enhance their service life. However, most of these techniques demand a high degree of pre-coating and post-coating operations that are often not cost effective. Moreover, these techniques produce large quantities of heat thereby forming heat-affected zones, which can lead to component warping, dimensional change and rejection of the components.
Size, shape and complexity of geometry of engineering components do restrict the applicability of thermal spray techniques. Moreover, these techniques require high quality and costly powders such as Tungsten Carbide-Cobalt, Chromium Carbide-Nickel Chrome, prepared by specially developed manufacturing routes such as atomization, fusing, sintering & crushing, chemical reduction and blending. Deposition efficiency of these powders is mostly less than 60%.
Hard facing is a term that refers to the deposition of filler metal on the surface of a work piece to improve its wear properties against abrasion, impact, erosion, galling & cavitation. It can also play an important role in enhancing the surface properties of a material to suit service conditions that impose an upper limit on the dimensions of a part. Cladding, surfacing, build up and buttering are some of the other categories that represent hard facing techniques.
Cladding is a process in which a relatively thick layer of filler metal is produced on a carbon or low-alloyed steel substrate to improve its corrosion resistance against such unfriendly atmospheres. Buildup refers to the addition of a weld metal to a base metal surface or to the edge of the joint or to a previously deposited weld metal for restoring the component dimensions to the required values. Buttering refers to the addition of one or more layers of a weld metal to the face of the joint or surface to be welded. Unlike build up, buttering is carried out for metallurgical reasons and not for dimensional control. Buttering is used especially for joining dissimilar metals and when stress relieving of the complete weld is not desirable.
Surfacing is generally used to improve, repair and rework a part so that it will have properties better than those of the actual part itself. In most of such cases, chemical compositions and mechanical properties have to be carefully considered, as they may be different for the surfacing material and the base material. Furthermore, dilution, which is defined as the percentage ratio of work piece melted to the total sum of the filler material and work piece material can also be an important consideration.
As most of the surface modification processes discussed above employ some means of heating, they lead to severe thermal stresses, which may warp or damage the work piece and thereby damage the surface produced. In addition to the above considerations, all these surface modification processes must be economical and be capable of being carried out in house.
In order to avoid the major problems associated with the above processes without sacrificing the advantages associated with welding, spark coating has been developed. This is also known by other names such as “Spark Alloying”, “Spark Hardening”, “Spark Toughening”, “Pulsed Electrode Surfacing” and “Electrospark Deposition”.
The Electro Spark Coating (ESC) method is well known and the essence of this method is as follows: Current pulses are generated between a processing electrode and the work piece, which are periodically brought in contact with each other. As they approach each other, at a particular moment, the breakdown of the inter electrode gap takes place and an electric discharge is produced, through which the energy stored in capacitors is released. This spark discharge results in erosion and results in the transfer of the material of the processing electrode onto the work piece i.e. it produces a surface coating and forms a modified layer on the surface of the work piece.
By sweeping the electrode over a selected zone of the work piece, a metallurgical modification such as surface alloying or hardening of the selected area in the work piece can be obtained. In this technique, both the consumable electrode i.e. coating material and the job i.e. work piece, should be electrically conductive as both of them are connected to the positive and negative terminals of a DC supply. The material to be deposited forms an electrode, which is mounted on an electromagnetic vibrator. The vibrations that are generated by the electromagnetic vibrator produce the periodic make and break of the contact between the consumable electrode and the work piece, which therefore initiate and define the rate and frequency at which the pulse discharges occur. These vibrations that are generated at the electromagnetic vibrator also prevent welding of the consumable electrode that is mounted on it to the work piece.
Auxiliary movements such as rotation of the electrode about its axis and a combination of rotation and linear vibrations have also been explored.
The phenomena of transferring a consumable electrode material on to a substrate material by means of short duration electrical discharges has been carried out in a variety of ways over several decades. The following references are representative of such coating processes.
The prior art referred to in the U.S. Pat. No. 4,405,851, year 1983, discloses a common method for depositing hard anode material on to the cathode work piece through electro sparking. The device of the prior art works on a resistance-capacitance (RC) relaxation circuit shown in FIG. 1, employing a vibratory electrode holder (1) to make and break the contact between an anode (4) (consumable electrode) and a cathode (2) (work piece) at regular intervals. When direct current DC (6) is supplied to the circuit resulting sparks between anode cathode pair melts and spray-deposits, part of the electrode (4) on to the work piece (2). A capacitor (3) is used in parallel with a direct current source (6) for producing a high-energy short duration spark. A ballast (5) is connected between the positive lead of the circuit and the electrode (4). The negative lead of the circuit is connected to the work piece (2).
It was assumed that the molten metal produced from the high temperature spark is transferred from the anode (4) to the cathode work piece (2) of the circuit possibly by an expanding gas bubble. Although similar to welding, this material transfer involves a complex mechanism. During the process of material transfer, the material reacts with the atmosphere where the coating is being applied e.g. nitrogen and oxygen, from the air in the atmosphere. The desired high-energy short duration spark is produced by connecting a capacitor (3) in parallel with the direct current source (6). The value of the resistor (5) must be large enough to prevent arcing after breakdown (capacitor discharge) has been initiated across the gap. A large resistance prevents arcing but reduces the maximum energy that can be released from the capacitor. This is the inherent difficulty with this process of depositing thicker coatings.
Further disclosure in a modified circuit of their invention in U.S. Pat. No. 4,405,851, shows that their circuit can work with either a single phase or a three-phase alternating current supply. Instead of a ballast or load resistor (5) as shown in FIG. 1, a high-speed thyristor switch (10) is used to control the capacitor discharge. High frequency switching can be used to open and close the capacitor-work piece-electrode circuit. The circuit shown in FIG. 2 includes a clamp (13) for holding and oscillating the anode (12) about a central axis on the work piece (14). The switching device (9) is also provided for repeatedly switching the circuit on & off at a predetermined rate. An independent trigger pulse generator is supplied & connected to the thyristor for producing selectively spaced and controlled pulses in series with the positive lead. In this circuit, the capacitance value is reported to be in the range of 2 to 800 μF. Two Diodes (7) & (8) are used to bypass and short out the reverse current through the electrode-work piece contact area & connecting leads and protect the thyristor against reversals.
The device shown in FIG. 3 uses a current transformer (15) with primary winding connected to a three-phase alternating current supply. The secondary winding is connected to a variable transformer (16) to control the voltage at electrode [anode] (21). The Secondary winding is connected to the rectifier (R) that provides the current for charging the single storage capacitor. The outputs of all the three rectifiers are connected in parallel to the single capacitor (3).
The voltage requirement of the circuits shown in FIG. 2 and FIG. 3 range from 60V to 120V and these devices produce coatings with a surface roughness of 40 μm Ra or more. The working voltage range, mentioned above, is hazardous for the operator and precautions must be taken to avoid accidents. Likewise, the surface roughness values are un-acceptable for most applications in an as-coated condition. Although finishing operations such as polishing and buffing can reduce the roughness, the consequent reduction in the coating thickness renders them unsuitable for any application.
U.S. Pat. No. 4,346,281, year 1982, discloses a method and apparatus for the surface treatment of metallic work pieces using a multiple electrode-rotating tool. The difference between the device of this patent as compared to the device mentioned above is the use of multiple anodes.
Slapping contact of each of the electrode with work piece results in spark discharge and hence coating when both of them are connected to the positive and negative poles of the direct current power supply. The electrode should be less than 2 mm in diameter and the length up to 8 mm. Chance of electrode fracture is very high as the speed range is very high from 50 to 20,000 rpm. Spark duration was 60 microseconds and coatings were deposited with the help of rotating tool electrode holder. Working voltage was not mentioned while deposition rate mentioned was 22 mg/min.
U.S. Pat. No. 4,866,237, year 1989, discloses another apparatus and a method where longitudinal vibration, in addition to travel of the electrode along a path, which is not perpendicular to but has a proper slant angle with the work piece, was provided in order to get a motion of slow approach and subsequent moving away of the electrode (anode) from the work piece. Capacitance values were not mentioned whilst the working voltage was mentioned as 50 V, pulse duration 200 microseconds and coating thickness was limited to less than 50 micrometers. Tubular consumable electrode was employed for obtaining maximum deposition.
U.S. Pat. No. 5,448,035, year 1995, discloses yet another method and apparatus for pulse fusion surfacing where the electrode holder rotates and oscillates the electrode simultaneously to increase the quantity of the deposit. The spark rate was varied according to various process parameters. Thyristor fired trigger/discharge resistance capacitance circuit and a stepper motor were controlled by a microprocessor. The thyristor-controlled circuit works at a voltage range of 40-70 V. Single pulse train produces coatings whose thickness was limited to 25 micrometers with tungsten carbide cobalt electrode. Oscillating frequency, rotational direction and speed of electrode can be controlled to achieve coatings with predetermined morphology and thickness on work piece.
U.S. Pat. No. 5,980,681, year 1999, describes a process for treatment of metal work piece surface by electrical discharges. The process was designed in such a way that microscopic melting of the work piece surface was avoided while ensuring extinction of the individual are discharges. This process was employed for roughening a metal work piece surface. Pulses of 50 A current and 20 micro second duration were employed.
U.S. Pat. No. 6,020,568, year 2000, describes an electromechanical process and apparatus for metal deposition using a direct current source. This apparatus basically includes an ultrasonic generator, a work piece holder, mounting attachments for holding the material to be coated on ultrasonic horn. The main difference being the use of ultrasonic generator instead of vibrating electrode holder. The coating thickness was limited by 5 micrometers/pass. The frequency of ultrasound may be in the range of 10 kHz to 40 kHz.
U.S. Pat. No. 6,417,477 B1, year 2002, discloses a method and apparatus for Electrospark alloying. Externally cooled special collet was incorporated to hold the consumable electrode for improving the deposition rate. While working voltage was 120 V, total capacitance value mentioned was 40 microfarads. A rotating tool electrode holder was employed for depositing the coatings in controlled atmosphere such as argon and vacuum. The deposition rate was mentioned to be 2.5 mg/min in air while the same is reduced by ⅓ in controlled atmosphere.
U.S. Pat. No. 6,835,908 B2, year 2004, describes still another method and apparatus for controlling Electrospark deposition using electrical variable wave forms from the Electrospark deposition process as a feed back parameters for optimizing the contact force between consumable anode and work piece cathode. The plurality of amplitudes of series of electrical energy pulses delivered to consumable electrode and work piece pair were measured and correlated to the contact force between electrode tip and work piece. With such set up it was claimed the flawed areas such as pits and groves can also be effectively coated. However, deposition rate and other electrical parameters were not revealed.
To sum up, the above prior art methods and apparatus employ higher working voltages ranging from 60 to 120 V or even more, a single pulse train (one pulse with a fixed width), lower pulse durations (less than 100 μsecond), higher current and lower capacitance values and thyristors or thyratrons as the pulses switching devices. All these factors lead to lower deposition rates and higher coating roughness. It is also clear from the above prior art information that the coating thickness is limited to at the most 50 microns when carbide materials are deposited on steel substrates. Moreover, the improvement in performance parameters due to these coatings is reported to be less than or equal to 500 times that of the bare sample when tested in pin on disk test rig. The resulting coatings are also not found to be uniform.
In summary the conventional spark deposition, whether they are vibratory, rotatory or multi electrode holder, are undesirably limited to achieve satisfactory results with regards to the thickness of the coating/deposit, rate of deposition and the treated surface roughness and uniformity of the deposited layer.
In spite of the wide variety of earlier processes, there is still scope for improving the thickness and reducing the surface roughness of the Electrospark coatings that can be deposited on any electrically conducting work pieces for example steels, their alloys and super alloys so as to increase the spectrum of application using this technique.
The main drawback of the earlier devices was that the roughness of the processed surface is a sizable fraction, ¼ to ½ or more, of the average thickness of the total deposited coated layer, which results in a poorer quality of the obtained surface. Moreover, the earlier devices do not permit a uniform increase in the thickness of the deposited layer as additional pulses can only result in the generation of sparks from the peaks of the previously deposited top layer, which can worsen the surface roughness even more.
The present invention is directed towards improving the thickness and reducing the surface roughness of the Electrospark coatings that can be deposited on any electrically conducting work pieces as well eliminating the difficulties of the prior art devices.
Considering these objectives and to meet the present day need for coatings with improved coating thickness, and better tribological, electrical and wear resistance properties, research work for developing improved Electrospark coating methods and devices has gained importance.